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Risky Business: Gambling on Climate Sensitivity

Posted on 21 September 2010 by gpwayne

There are some things about our climate we are pretty certain about. Unfortunately, climate sensitivity isn’t one of them. Climate sensitivity is the estimate of how much the earth's climate will warm if carbon dioxide equivalents are doubled. This is very important because if it is low, as some sceptics argue, then the planet isn’t going to warm up very much. If sensitivity is high, then we could be in for a very bad time indeed.

There are two ways of working out what climate sensitivity is (a third way – waiting a century – isn’t an option, but we’ll come to that in a moment). The first method is by modelling:

Climate models have predicted the least temperature rise would be on average 1.65°C (2.97°F) , but upper estimates vary a lot, averaging 5.2°C (9.36°F). Current best estimates are for a rise of around 3°C (5.4°F), with a likely maximum of 4.5°C (8.1°F).

The second method calculates climate sensitivity directly from physical evidence:

These calculations use data from sources like ice cores, paleoclimate records, ocean heat uptake and solar cycles, to work out how much additional heat the doubling of greenhouse gases will produce. The lowest estimate of warming is close to the models - 1.8°C (3.24°F ) on average - but the upper estimate is a little more consistent, at an average of around 3.5°C (6.3°F).

It’s all a matter of degree

To the lay person, the arguments are obscure and complicated by other factors, like the time the climate takes to respond. But climate sensitivity is not just an abstract exchange of statistics relevant only to scientists. It also tells us about the likely changes to the climate that today's children will inherit.

Consider a rise in sea levels, for example. Predictions range from centimetres to many metres, and the actual increase will be governed by climate sensitivity. The 2007 IPCC report proposed a range of sea level rises based on different increases in temperature, but we now know they underestimated sea level rise, perhaps by a factor of three, in part because of a lack of data about the behaviour of Greenland and Antarctic ice-sheets.

Current estimates of sea level rise alone, as a result of a two degree rise in temperature, are very worrying. More worrying is that the current projections do not account for recently accelerated melting of polar regions. There are also many other possible effects of a 2°C rise (3.6°F) that would be very disruptive.

All the models and evidence confirm a minimum warming close to 2°C for a doubling of atmospheric CO2 with a most likely value of 3°C and the potential to warm 4.5°C or even more. Even such a small rise would signal many damaging and highly disruptive changes to the environment. In this light, the arguments against mitigation because of climate sensitivity are a form of gambling. A minority claim the climate is less sensitive than we think, the implication being we don’t need to do anything much about it. Others suggest that because we can't tell for sure, we should wait and see.

In truth, nobody knows for sure quite how much the temperature will rise, but rise it will. Inaction or complacency heightens risk, gambling with the entire ecology of the planet, and the welfare of everyone on it.

This post is the Basic version (written by Graham Wayne) of the skeptic argument "Climate sensitivity is low". For the stout of heart, be sure to also check out the Advanced Version by Dana which is currently getting rave reviews on Climate Progress.

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Comments 1 to 7:

  1. Good post but in my opinion it needs a last sentence, along the lines of "remedial measures should therefore be adopted befor it is too late."
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  2. I thought the last sentence was already pretty good. It really doesn't need the "before it is too late" kind of thing huntjanin proposes. In fact, I was pretty pleased with the way it built up carefully to the pivotal and climactic statements, "Even such a small rise would signal many damaging and highly disruptive changes to the environment. In this light, the arguments against mitigation because of climate sensitivity are a form of gambling." That is the essential point of the post: it is a form of gambling, and a particularly foolish one, like that game 'chicken' middle schoolers play -- but on a planetary scale. In fact, if I were to ask to any change at all, it would be right here, rather than at the end: something to make it clear that we are not talking about just any 'gambling', but such a particularly reckless one, one where the odds are not only heavily against us, but the price paid is unimaginably high compared to any other form of gambling. So I might, for example, change "a form of gambling" to "the worst sort of gamble, with planetary disaster for the price of losing."
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  3. I thought the entire article was well written, Graham, thanks. Regarding the above comments on gambling, and stressing the potential disastrous outcomes - I guess it's a fine line to be walked, between over-caution on the one hand, which may lead to complacency, and alarmism on the other, which will of course lead to dismissal by the denialosphere. (Mind you, the skeptic tendency to leave zero space between 'alarmism' and 'nothing to worry about' makes that a particularly difficult task...) If the more pessimistic results from the climate models are at all correct, it's not just gambling - it's more like playing russian roulette, but with nuclear weapons... if you're wrong, it doesn't just affect you, it affects everyone else as well. Sure, it may just be a sub-kiloton tactical nuke, that 'only' takes out your neighbourhood. Then again, it might be the launch code for the world's entire arsenal. The sad bit is that some people look at that problem, and see that it might not be their neighbourhood, but, say, a neighbourhood in Bangladesh, or a low-lying island nation, and they think "Meh, it doesn't affect me" and push the button anyway...
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  4. Apart from the die-hard skeptics, I think that for the general public is not even a gambling. My impression is that our time horizon is pretty near, we're not able (or not willing) to consider long term plans. In this sense we are culturally limited to the financial-style next quarter results. These considerations does not apply just to climate or global resources limitation but even to our everyday life. And it's the prologue for the disaster.
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  5. Riccardo "My impression is that our time horizon is pretty near, we're not able (or not willing) to consider long term plans." I really think it's a failure of the imagination. I can speculate about my grandchildren's grandchildren. When people talk about this they don't really visualise it, they just hope that they'll be decent people who'll enjoy life in much the same way we do. In this I'm more with Hansen. I see good possibilities if this generation and the next does the right thing. I see nasty stuff if we and they don't. Of course for them, it'll be just the way the world is. But my generation is not too thrilled about the way our predecessors mucked up our forests and agricultural lands and introduced pests of every kind. I'd hope that our descendants will see that we did our best even though it was too little too late for some things they'll never get to see.
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  6. A few little things that struck me: It seems a little odd to include Royer 2007 in the climate model category, it's a bit of a hybrid but based on past changes in CO2. Also, where does that the top limit for Royer come from? The paper states "GEOCARBSULF simulations cannot exclude the possibility of a high climate sensitivity"? Finally, why not include Lunt 2010 and Pagani 2010?
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  7. adelady, maybe i looked pessimistic while i'm not. What I've tryed to say is that it's a broader cultural problem and consequently broader changes in our societies are required. In this, science alone is not enough.
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